The plug for piercing and rolling is used to pierce and roll a heated round billet of a metal material and make it into a metal pipe (seamless pipe). The plug is arranged on the pass line of a piercing mill and penetrates through the billet along the central axis of the billet rotated in the circumferential direction by two inclined rolls opposed to each other with the pass line therebetween. At the time, the plug contacts the billet and receives heat and stress from the billet, and therefore its surface is prone to wear and dissolution.
One approach to prevent the wear and dissolution of the plug surface is to form an oxide scale layer having a thickness of about several hundred micrometers on the plug surface. The oxide scale layer having good wettability and adiabaticity can therefore reduce the wear and dissolution of the plug surface.
The oxide scale layer formed on the plug surface is however sometimes partly peeled off during piercing and rolling. If the oxide scale layer is thus peeled off, the plug ends up having irregularities on the surface. The irregularities are transferred onto the inside surface of a billet in the process of being pierced and rolled. As the result, the metal pipe obtained after the piercing and rolling has defects on its inside surface.
The inventors have proposed a plug used to solve the problem in the disclosure of Japanese Patent No. 3777997. The oxide scale layer formed on the plug surface by thermally treating the plug includes inner scale layer formed on the surface of the plug material and outer scale layer formed on the inner scale layer. The inner scale layer having a dense structure is less easily peeled off. On the other hand, the outer scale layer having a porous structure is more easily peeled off than the inner scale layer. Therefore, according to the Patent Document, the outer scale layer is removed in advance and the plug having the inner scale layer remaining thereon is used for piercing and rolling. The inner scale layer having a dense structure is less easily peeled off than the outer scale layer, and therefore inside surface defects during piercing and rolling can be reduced, so that the wear and dissolution of the plug can be reduced.
While the outer scale layer is more easily peeled off than the inner scale layer, a high load must be applied on the outer scale layer in order to remove the outer scale layer in advance. For example, as disclosed by the Patent Document, the outer scale layer is provided with high impact force using a hammer or the like. Alternatively, the outer scale layer surface must be provided with rapid thermal stress by rapidly heating the surface of the outer scale layer using a burner. The task of removing the outer scale layer includes a large workload. In order to use the plug disclosed by Patent Document for the manufacture of a metal pipe, the outer scale layer must readily be removed.
Note that another prior art document relevant to the present application is JP 8-206709 A.